Low Friction Centralizer

ABSTRACT

A centralizer for a downhole system, such as but not limited to a casing system. In some embodiments, the centralizer has a tubular body and a plurality of roller ball assemblies circumferentially spaced about the tubular body. Each roller ball assembly includes a plurality of rotatable balls adapted to engage a surface radially offset from the centralizer and rotate relative to the surface in any direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional application Ser. No. 61/174,617 filed May 1, 2009 and entitled “Low Friction Centralizer,” which is hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The disclosure relates to centralizers for downhole tubulars, such as casing strings. More particularly, the disclosure relates to a centralizer having roller balls that facilitate movement of the centralizer and a casing string coupled thereto relative to a surrounding formation or another casing string.

Centralizers are commonly used during completions operations in a wellbore, such as to cement a casing string within the wellbore. Prior to installation of the casing string within the wellbore, a centralizer is positioned within or about the casing string. The casing string with the centralizer coupled thereto is then lowered into the wellbore. As the casing string is lowered, the centralizer contacts the surrounding formation. Contact between the centralizer and formation impedes movement of the casing string and thus its installation. After the casing string is positioned within the wellbore, the centralizers maintain the casing string at the wellbore center, allowing cement to be uniformly distributed throughout an annulus formed by the casing string and surrounding formation.

To reduce frictional loads resulting from contact between the centralizer and formation during installation of the casing string, the centralizer typically has structural features that facilitate relative movement between the centralizer and formation. For instance, some conventional centralizers have raised vanes that enable sliding contact between the centralizer and formation over a limited area. Even so, slidingly engagement between the vanes and formation can generate significant friction loads. Other conventional centralizers have cylindrical rollers that rotatably engage the formation, resulting in comparatively lower frictional loads. However, movement of the centralizer is facilitated only in a single direction dependent upon the orientation of the rotational axis of the roller relative to the axial centerline of the centralizer. Movement of the centralizer in another direction causes the roller to slide against the formation, increasing frictional loads therebetween. Furthermore, the sliding engagement and associated frictional loads cannot be eliminated by the addition of other rollers having differently orientated rotational axes because at least one of the rollers will always slidingly engage the formation no matter what direction the centralizer moves.

Accordingly, there is a need for a centralizer that facilitates movement between the centralizer and casing string coupled thereto relative to the formation, or another casing string, in any direction with reduced associated frictional loads.

SUMMARY OF THE DISCLOSURE

A centralizer for a downhole system, including but not limited to a casing system, is disclosed. In some embodiments, the centralizer has a tubular body and a plurality of roller ball assemblies circumferentially spaced about the tubular body. Each roller ball assembly includes a plurality of rotatable balls adapted to engage a surface radially offset from the centralizer and rotate relative to the surface in any direction.

In some embodiments, the system includes a tubular positioned in a wellbore and a centralizer supported by the tubular. The centralizer has a roller ball assembly with a plurality of balls engaging a surface radially offset from the centralizer and rotatable over the surface in any direction.

In some embodiments, the system includes two concentric tubulars positioned in a wellbore, the two concentric tubulars comprising an inner tubular and an outer tubular, and a centralizer disposed therebetween. The centralizer includes a plurality of balls engaging the tubulars and rotatable in any direction. Rotation of the balls enables relative movement of the tubulars.

Thus, embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with conventional centralizers. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the disclosed embodiments, reference will now be made to the accompanying drawings in which:

FIG. 1 is schematic representation a low friction centralizer in accordance with the principles disclosed herein positioned in a casing string suspended in a wellbore;

FIG. 2 is a perspective view of the centralizer of FIG. 1;

FIGS. 3A and 3B are axial and radial cross-sectional views, respectively, of the tubular body of FIG. 2;

FIGS. 4A and 4B are axial cross-sectional and top views, respectively, of the retainer plate of FIG. 2;

FIG. 5 is schematic representation of another embodiment of a low friction centralizer in accordance with the principles disclosed herein disposed about a casing string suspended in a wellbore;

FIG. 6 is a perspective view of the centralizer of FIG. 5;

FIG. 7 is a perspective view of the tubular body of FIG. 6;

FIG. 8 is a perspective view of the centralizer of FIG. 6 in partial cross-section;

FIGS. 9A and 9B are axial and radial cross-sectional views, respectively, of the ball socket block of FIG. 6;

FIGS. 10A and 10B are axial and radial cross-sectional views, respectively, of the retainer plate of FIG. 6;

FIG. 11 is schematic representation of yet another embodiment of a low friction centralizer in accordance with the principles disclosed herein rotatably disposed between two concentric casing strings;

FIG. 12 is a perspective view of the centralizer of FIG. 11;

FIG. 13 is a perspective view of the tubular body of FIG. 12;

FIG. 14 is a perspective view of the centralizer of FIG. 12 in partial cross-section;

FIGS. 15A and 15B are axial and radial cross-sectional views, respectively, of the ball socket block of FIG. 12; and

FIGS. 16A and 16B are axial and radial cross-sectional views, respectively, of the retainer plate of FIG. 12.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

The following description is directed to exemplary embodiments of a modular reinforce pipeline fabrication system and associated methods. The embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. One skilled in the art will understand that the following description has broad application, and that the discussion is meant only to be exemplary of the described embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and the claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. Moreover, the drawing figures are not necessarily to scale. Certain features and components described herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to. . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. Further, the terms “axial” and “axially” generally mean along or parallel to a central or longitudinal axis, while the terms “radial” and “radially” generally mean perpendicular to the central or longitudinal axis.

Referring now to FIG. 1, a schematic representation of a casing system 100, including a low friction centralizer 105 in accordance with the principles disclosed herein, is shown. Casing system 100 further includes an outer casing 110 installed within a wellbore 115 and an inner casing 120 suspended therein. As illustrated, outer casing 110 is secured in position by cement 130 disposed in an annulus 135 between outer casing 110 and a formation 140 surrounding wellbore 115. Inner casing 120 includes two casing pipe segments, or joints, 125 with centralizer 105 coupled therebetween. Centralizer 105 maintains inner casing 120 in a central position within outer casing 110 and enables movement of inner casing 120 relative to outer casing string 110, as will be described.

Turning next to FIG. 2, a perspective view of one centralizer 105 is shown. Centralizer 105 includes a tubular body 155 having a first end 160, a second end 165, and a flowbore 170 extending therethrough. At first end 160, centralizer 105 has external threads 175 that enable centralizer 105 to be threaded into a joint 125 (FIG. 1) of inner casing 120. At second end 165, centralizer 105 has internal threads 180 (FIG. 3A) that enable another joint 125 (FIG. 1) to be threaded into centralizer 105. When centralizer 105 is coupled between joints 125, as shown in FIG. 1, flowbore 170 enables conveyance of cement through centralizer 105 during cementing operations.

Centralizer 105 further includes a plurality of raised vanes 185 disposed circumferentially about tubular body 155. Each vane 185 has a length extending substantially in the longitudinal or axial direction and a height extending radially from the outer surface 190 of tubular body 155, thereby creating a valley 195 disposed between adjacent vanes 185. Referring now to FIGS. 3A and 3B, which depict axial and radial cross-sectional views, respectively, of tubular body 155, each vane 185 has a recess 200 therein. Recess 200 is bounded by radially extending surfaces 205 and an axially extending surface 210 therebetween. As best viewed in FIG. 3A, a plurality of ball receptacles 215 and fastener bores 220 are disposed in surface 210. Each ball receptacle 215 is defined by a spherical surface 225, whereas each fastener bore 220 is configured to receive a fastener, as will be described.

Returning briefly to FIG. 2, centralizer 105 further includes a roller ball assembly 230 coupled within recess 200 of each raised vane 185. Each roller ball assembly 230 includes a plurality of spherical balls 235, a plurality of fasteners 240, and a retainer plate 245. Each ball 235 is disposed within a ball receptacle 215 (FIG. 3A) of vane 185. Further, ball 235 is rotatable within ball receptacle 215 relative to vane 185 and thus tubular body 155 of centralizer 105 in all directions.

Turning to FIGS. 4A and 4B, top and axial cross-sectional views, respectively, of retainer plate 245 are shown. Retainer plate 245 includes a plurality of fastener throughbores 250 and a plurality of ball receptacles 255. Each fastener throughbore 250 is configured to receive a fastener 240 (FIG. 2) therethrough. Each ball receptacles 255 is bounded by a surface 260 configured to receive a ball 235 (FIG. 2). Surface 260 extends between a circular opening 265 in the inner surface 270 of retainer plate 245 and a circular opening 275 in the outer surface 280 of retainer plate 245. Opening 275 is defined by a diameter that is smaller than a diameter of each ball 235 (FIG. 2), whereas opening 265 is defined by a diameter that is at least that of the ball diameter.

To couple roller ball assembly 230 with tubular body 155 of centralizer 105, a ball 235 is disposed within each ball receptacle 215 of vane 185. Retainer plate 245 is then positioned over recess 200 of vane 185 such that ball receptacles 255 of retainer plate 245 align with and receive balls 235. Fasteners 240 are inserted through fastener throughbores 250 of retainer plate 245 and secured within fastener bores 220 of vane 185. In some embodiments, a lubricant is injected within ball receptacles 215 of vanes 185 and/or ball receptacles 255 of retainer plate 245 prior to coupling of retainer plate 245 to vane 185 to promote rotation of balls 235 relative to vanes 185 and retainer plate 245 for extended periods of time.

When retainer plate 245 is coupled to vane 185, as described, balls 235 are retained by retainer plate 245 within recess 200 because openings 275 have diameters smaller than those of balls 235. The height of recess 200 and the depths of ball receptacles 215, 255, each dimension measured in the radial direction, are selected such that a portion of each ball 235 extends radially through its respective opening 275 in retainer plate 245 and beyond outer surface 280 of retainer plate 245. As such, balls 235 engage outer casing 110 (FIG. 1).

When inner casing 120 is disposed within outer casing 110, such as during installation of inner casing 120, contact between balls 235 and outer casing 110 causes rotation of balls 235 within ball receptacles 215 of vanes 185. Thus, balls 235 of centralizer 105 rotatably engage outer casing 110. Because balls 235 may freely rotate in any direction, friction loads associated with such contact are greatly reduced in comparison to those associated with conventional centralizers, including those previously described. In other words, centralizers 105 facilitate low friction, or near unimpeded, movement of inner casing 120 relative to outer casing 110 regardless of its direction of movement.

In the above-described embodiment, centralizer 105 is coupled between joints 125 of inner casing 120, and thus is integral to inner casing 120. In other embodiments, the low friction centralizers are not integral to a casing but are instead “slipped on” and coupled to its exterior surface. FIGS. 5-8 illustrate an embodiment of a low friction, slip-on centralizer.

Beginning with FIG. 5, a schematic representation of a casing system 300, including a low friction centralizer 305 in accordance with the principles disclosed herein, is shown. Casing system 300 further includes an outer casing 310 installed within a wellbore 315 and an inner casing 320 suspended therein. Outer casing 310 is secured in position by cement 330 disposed in an annulus 335 between outer casing 310 and a formation 340 surrounding wellbore 315. Inner casing 320 includes two casing pipe segments, or joints, 325 threaded end-to-end. Centralizer 305 is installed about inner casing 320 to maintain inner casing 320 in a central position within outer casing 310 and to enable movement of inner casing 320 relative to outer casing string 310, as will be described.

Turning to FIG. 6, a perspective view of one centralizer 305 is shown. Centralizer 305 includes a tubular body 325 and a plurality of roller ball assemblies 330 disposed circumferentially thereabout. Tubular body 325 has a first end 335, a second end 340, a throughbore 345 extending therethrough, and a plurality of circumferentially spaced bores 355 proximal ends 335, 340. Throughbore 345 enables centralizer 305 to be positioned about, or “slipped on,” inner casing 320, as illustrated in FIG. 5. Each bore 355 is configured to receive a fastener 360 to enable coupling of centralizer 305 about inner casing 320. When secured to inner casing 320 via fasteners 360, centralizer 305 does not move appreciably relative to inner casing 320. For this reason, centralizer 305 may be referred to as a “fixed, slip-on centralizer.” Tubular body 325 further includes a plurality of circumferentially spaced cutouts 350, best viewed in FIG. 7. Each cutout 350 is configured to receive a roller ball assembly 330 therein, as will be described.

Referring now to FIG. 8, a perspective view of centralizer 305 is shown in partial cross-section to better illustrate features of a roller ball assembly 330. As shown, roller ball assembly 330, depicted in cross-section, is positioned within a cutout 350 of tubular body 325. Roller ball assembly 330 includes a ball socket block 365, a retainer plate 370, and a plurality of spherical balls 375 and fasteners 380 extending therebetween.

Turning to FIGS. 9A and 9B, top and axial cross-sectional views, respectively, of ball socket block 365 are shown. Ball socket block 365 has an outer surface 385 with a plurality of ball receptacles 390 and fastener bores 395 disposed therein. Each fastener bore 395 is configured to receive a fastener 380 (FIG. 8) to enable coupling of retainer plate 370 thereto. Each ball receptacle 390 is defined by a spherical surface 400 configured to receive a ball 375 (FIG. 8). As such, ball socket block 365 effectively performs the same function as vanes 185 of centralizer 105, previously described.

FIGS. 10A and 10B are similar views of retainer plate 370. As shown, retainer plate 370 includes a plurality of fastener throughbores 400 and a plurality of ball receptacles 405. Each fastener throughbore 400 is configured to receive a fastener 380 (FIG. 8) therethrough. Each ball receptacle 405 is bounded by a surface 410 extending between a circular opening 415 in the inner surface 420 of retainer plate 370 and a circular opening 425 in the outer surface 430 of retainer plate 370. Opening 425 is defined by a diameter that is smaller than a diameter of each ball 375 (FIG. 8), whereas opening 415 is defined by a diameter that is at least that of the ball diameter.

To couple roller ball assembly 330 with tubular body 325 of centralizer 305, ball socket block 365 is disposed within cutout 350 of tubular body 325, as shown in FIG. 8, and welded, or otherwise secured, to tubular body 325. Next, a ball 375 is disposed within each ball receptacle 390 of ball socket block 365. Ball 375 is freely rotatable within ball receptacle 390 relative to ball socket block 365 in all directions. Retainer plate 370 is then positioned over ball socket block 365 such that ball receptacles 405 in retainer plate 370 align with and receive balls 375. Lastly, fasteners 380 are inserted through fastener throughbores 400 of retainer plate 370 and secured within aligned fastener bores 395 in ball socket block 365. In some embodiments, a lubricant is injected within ball receptacles 390 of ball socket block 365 and/or ball receptacles 405 of retainer plate 370 prior to coupling of retainer plate 370 to ball socket block 365 to promote rotation of balls 375 relative to ball socket block 365 and retainer plate 370 for extended periods of time.

When retainer plate 370 is coupled to ball socket block 365, as described, balls 375 are retained therebetween because openings 425 of retainer plate 370 have diameters smaller than those of balls 375. The depths of ball receptacles 390, 405, each dimension measured in the radial direction, are selected such a portion of each ball 375 extends radially through its respective opening 425 in retainer plate 370 and beyond outer surface 430 of retainer plate 370. As such, balls 375 engage outer casing 310 (FIG. 5).

When inner casing string 320 moves within outer casing 310, such as during installation of inner casing 320, contact between balls 375 and outer casing 310 causes rotation of balls 375 within roller ball assembly 330. Thus, balls 375 of centralizer 305 rotatably engage outer casing 310. Because balls 375 may freely rotate in any direction, friction loads associated with such contact are greatly reduced in comparison to those associated with conventional centralizers, including those previously described. In other words, centralizer 305 facilitates low friction, or near unimpeded, movement of inner casing 320 relative to outer casing 310 regardless of its direction of movement.

In the previously described embodiment, fixed, slip-on centralizer 305 does not move relative to inner casing 320. Even so, there may be instances where relative movement between centralizer 305 and inner casing 320 is desirable. FIGS. 11-16 illustrate an embodiment of a low friction, slip-on centralizer that permits such movement.

Beginning with FIG. 11, a schematic representation of a casing system 600, including a low friction centralizer 605 in accordance with the principles disclosed herein, is shown. Casing system 600 further includes an outer casing 610 installed within a wellbore 615 and an inner casing 620 suspended therein. Outer casing 610 is secured in position by cement 630 disposed in an annulus 635 between outer casing 610 and a formation 640 surrounding wellbore 615. Inner casing 620 includes two casing pipe segments, or joints, 625 threaded end-to-end.

Centralizer 605 is installed about inner casing 620 to maintain inner casing 620 in a central position within outer casing 610. Further, centralizer 605 is moveable relative to outer casing 610 and to inner casing 620. To maintain the axial position of centralizer 605 relative to inner casing 620, casing system 600 further includes two locking collars 645 coupled to inner casing 620 above and below centralizer 605. Locking collars 645 do not move relative to inner casing 620 and thereby limit movement of centralizer 605 in the axial direction relative to inner casing 620.

Referring to FIG. 12, a perspective view of one centralizer 605 is shown. Centralizer 605 includes a tubular body 610 and a plurality of roller ball assemblies 615 disposed circumferentially thereabout. Tubular body 610 has a first end 620, a second end 625, and a throughbore 630 extending therethrough. Throughbore 630 enables centralizer 605 to be positioned about, or “slipped on,” inner casing 620, as illustrated in FIG. 11. Unlike centralizer 305, previously described, tubular body 610 is not fastened to inner casing 620, but rather is enabled by roller ball assemblies 615 to move relative to inner casing 620. Thus, tubular body 610 does not include fastening means, such as fastener bores proximal ends 620, 625. Because centralizer 605 is moveable relative inner casing 620 but restricted by locking collars 645 (FIG. 11) from moving appreciably in the axial direction relative to inner casing 620, centralizer 605 may be referred to as a “rotatable, slip-on centralizer.” Tubular body 610 further includes a plurality of circumferentially spaced cutouts 635, best viewed in FIG. 13. Each cutout 635 is configured to receive a roller ball assembly 615 therein, as will be described.

Referring now to FIG. 14, a perspective view of centralizer 605 is shown in partial cross-section to illustrate features of a roller ball assembly 615. As shown, roller ball assembly 615, depicted in cross-section, is positioned within a cutout 635 of tubular body 610. Roller ball assembly 615 includes a ball socket block 640, a retainer plate 645, and a plurality of spherical balls 650 and fasteners 655 extending therebetween. In contrast to retainer plate 370 of centralizer 305, retainer plate 645 is disposed radially inward of ball socket block 640 and coupled thereto by fasteners 655 extending from the interior of centralizer 605. In the unlikely event that fasteners 655 were to loosen, inner casing 620 (FIG. 11) would prevent them from disengaging retainer plate 645.

Turning to FIGS. 15A and 15B, top and axial cross-sectional views, respectively, of ball socket block 640 are shown. Ball socket block 640 has an inner surface 660 with a plurality of ball receptacles 665 and fastener bores 670 disposed therein. Each fastener bore 670 is configured to receive a fastener 655 (FIG. 14) therein. Each ball receptacle 665 is defined by a spherical surface 675 configured to receive a ball 650 (FIG. 14). Surface 675 extends between a circular opening 680 in inner surface 660 and a circular opening 685 in the outer surface 690 of ball socket block 640. Opening 685 is defined by a diameter that is smaller than a diameter of each ball 650 (FIG. 8), whereas opening 680 is defined by a diameter that is at least that of the ball diameter.

FIGS. 16A and 16B are similar views of retainer plate 645. As shown, retainer plate 645 includes a plurality of fastener throughbores 695 and a plurality of ball receptacles 700. Each fastener throughbore 695 is configured to receive a fastener 655 (FIG. 8) therethrough. Each ball receptacle 700 is defined by a spherical surface 705 configured to receive a ball 650 (FIG. 14). Surface 705 extends between a circular opening 710 in the inner surface 715 of retainer plate 645 and a circular opening 720 in the outer surface 725 of retainer plate 645. Opening 710 is defined by a diameter that is smaller than a diameter of each ball 650 (FIG. 14), whereas opening 720 is defined by a diameter that is at least that of the ball diameter.

To couple roller ball assembly 615 with tubular body 610 of centralizer 605, a ball 650 is disposed within each ball receptacle 665 of ball socket block 640. Retainer plate 645 is then positioned over ball socket block 640 such that ball receptacles 700 of retainer plate 645 align with and receive balls 650. Fasteners 655 are inserted through fastener throughbores 695 of retainer plate 645 and secured within aligned fastener bores 670 in ball socket block 640. In some embodiments, a lubricant is injected within ball receptacles 665 and/or ball receptacles 700 prior to coupling of retainer plate 645 to ball socket block 640 to promote rotation of balls 650 relative to ball socket block 640 and retainer plate 645 for extended periods of time. Lastly, roller ball assembly 615 is disposed within cutout 635 of tubular body 610, as shown in FIG. 14, and secured in position, such as by welding ball socket block 640 to tubular body 610.

When retainer plate 645 is coupled to ball socket block 640, as described, balls 650 are retained therebetween because openings 710 of retainer plate 640 and openings 685 of ball socket block 640 have diameters smaller than those of balls 650. At the same time, balls 665 are freely rotatable within ball receptacles 665, 700 relative to ball socket block 640 and retainer plate 645 in all directions. The thickness of ball socket block 640 between surfaces 660, 690 and the thickness of retainer plate 640 between surfaces 715, 725 are selected such that a portion of each ball 650 extends radially through ball receptacle 700 in retainer plate 645 and beyond inner surface 715 of retainer plate 645. Similarly, a portion of each ball 650 extends radially through ball receptacle 665 in ball socket block 640 and beyond outer surface 690 of ball socket block 640. As such, balls 650 engage inner casing 620 and outer casing 610 (FIG. 11).

When inner casing 620 moves within outer casing 610, such as during installation of inner casing 620, contact between balls 650 and casings 610, 620 causes rotation of balls 650 within roller ball assembly 615. Thus, balls 650 of centralizer 615 rotatably engage outer casing 610 and inner casing 620. Because balls 650 may freely rotate in any direction, friction loads associated with such contacts are greatly reduced in comparison to those associated with conventional centralizers, including those previously described. In other words, centralizer 605 facilitates low friction, or near unimpeded, movement of inner casing 620 relative to outer casing 610 regardless of its direction of movement.

Furthermore, balls 650 facilitate low friction movement of centralizer 605 relative to inner casing 620 in any direction. This may be particularly useful in other embodiments wherein outer casing 610 is not fixed, but is moveable like inner casing 620. In the illustrated embodiment, however, locking collars 645 (FIG. 11) limit axial movement of centralizer 605 relative to inner casing 620.

As described, centralizer 305 has a tubular body 325 with a plurality of cutouts 350, each cutout 350 receiving a ball socket block 365, which is coupled to tubular body 325, such as by welding. Similarly, centralizer 605 has a tubular body 610 with a plurality of cutouts 635, each cutout 635 receiving a ball socket block 640, which is coupled to tubular body 610, such as by welding. One of ordinary skill in the art will readily appreciate that tubular body 325, 610 and ball socket block 365, 640, respectively, may be formed integrally as a single component, rather than as separate components subsequently joined in some manner. For example, tubular body 325 and ball socket block 365 may be formed as a single component through casting or forging. During assembly of centralizer 305, balls 375 would then be seated in ball receptacles 390 of the integral tubular body and ball socket block and retainer plate 370 coupled thereto. Likewise, tubular body 610 and ball socket block 640 may be formed as a single component through casting or forging. During assembly of centralizer 605, balls 650 would then be seated in ball receptacles 665 of the integral tubular body and ball socket block and retainer plate 645 coupled thereto.

A centralizer in accordance with the principles disclosed herein, including the embodiments described above, enables low friction movement of the centralizer relative to a downhole tubular, such as a casing string, or a surrounding formation. Movement of the centralizer relative to the casing string, or surrounding formation, is facilitated by a plurality of balls which engage the casing string, or formation, and rotate freely in any direction. Thus, the centralizer is moveable in any direction relative to the casing string or formation. The friction forces associated with such movement are no greater in one direction than any other, in contrast to many conventional centralizers. Moreover, the friction forces are significantly less than those associated with many conventional centralizers, in particular those which enable sliding engagement, as previously described.

While various embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings herein. The embodiments herein are exemplary only, and are not limiting. Many variations and modifications of the apparatus disclosed herein are possible and within the scope of the invention. For example, centralizers 105, 305, 605 are depicted and described as facilitating movement of an inner casing 120, 320, 620 within a fixed outer casing 110, 310, 610, respectively. Centralizers 105, 305, 605 would function identically as described were they to instead engage a surrounding formation 140, 340, 640 in the absence of outer casing 110, 310, 610. Furthermore, the embodiments of the low friction centralizers disclosed herein are described in the context of being integral with or coupled to a casing string for the purpose of centralizing the casing string and facilitating movement of the casing string relative to another casing string. One having ordinary skill in the art will readily appreciate that the low friction centralizers are equally applicable to other types of tubulars or tubular strings, such as but not limited to drill strings, which require centralization and/or movement relative to a formation or another tubular string. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. 

1. A centralizer for a downhole system comprising: a tubular body; and a plurality of roller ball assemblies circumferentially spaced about the tubular body, each roller ball assembly comprising: a plurality of rotatable balls adapted to engage a surface radially offset from the centralizer and rotate relative to the surface in any direction.
 2. The centralizer of claim 1, wherein the tubular body further comprises a plurality of circumferentially spaced raised vanes, each vane having a recess therein, the recess bounded by two radially extending surfaces and an axially extending surface therebetween.
 3. The centralizer of claim 2, wherein each vane comprises a plurality of ball receptacles in the axially extending surface, wherein one of the plurality of balls is rotatably seated in each ball receptacle.
 4. The centralizer of claim 3, further comprising a retainer plate coupled over each recess, the retainer plate having a plurality of ball receptacles, each ball receptacle aligned with a ball receptacle in the vane, and wherein one of the plurality of balls is rotatably disposed in each pair of aligned ball receptacles and extends through an opening in an outer surface of the retainer plate.
 5. The centralizer of claim 4, wherein the outer openings have a diameter less than that of the ball received therethrough.
 6. The centralizer of claim 1, wherein each roller ball assemblies comprises: a block having a plurality of ball receptacles; and a retainer plate coupled to the block, the retainer plate having a plurality of ball receptacles, each ball receptacle aligned with one of the plurality of ball receptacles of the block; wherein one of the plurality of balls is rotatably disposed in each pair of aligned ball receptacles.
 7. The centralizer of claim 6, wherein each roller ball assembly comprises an axially extending inner surface and an axially extending outer surface and wherein each ball extends through an opening in the outer surface.
 8. The centralizer of claim 7, wherein each ball extends through an opening in the inner surface.
 9. The centralizer of claim 8, wherein the openings have a diameter less than that of the ball extending therethrough.
 10. A system comprising: a tubular positioned in a wellbore; and a centralizer supported by the tubular, the centralizer comprising: a roller ball assembly having a plurality of rotatable balls engaging a surface radially offset from the centralizer and rotatable over the surface in any direction.
 11. The system of claim 10, wherein the radially offset surface is a surface bounding a formation surrounding the wellbore and wherein rotation of the balls enables movement of the centralizer relative to the formation.
 12. The system of claim 10, wherein the radially offset surface is a surface of another tubular disposed concentrically about the centralizer and wherein rotation of the balls enables movement of the centralizer relative to the another tubular.
 13. The system of claim 10, wherein the radially offset surface is a surface of the tubular disposed within the centralizer and wherein rotation of the balls enables movement of the centralizer relative to the tubular.
 14. The system of claim 10, wherein the centralizer is threaded to the tubular.
 15. The system of claim 10, wherein the centralizer is disposed about the tubular.
 16. The centralizer of claim 15, wherein the centralizer is coupled to the tubular, whereby the centralizer is immovable relative to the tubular.
 17. A system comprising: two concentric tubulars positioned in a wellbore, the two concentric tubulars comprising an inner tubular and an outer tubular; and a centralizer disposed therebetween, the centralizer comprising a plurality of balls engaging the tubulars and rotatable in any direction; wherein rotation of the balls enables relative movement of the tubulars.
 18. The centralizer of claim 17, wherein the centralizer further comprises a tubular body having a plurality of circumferentially spaced cutouts.
 19. The centralizer of claim 18, wherein, within each cutout, the centralizer further comprises: a block coupled to the tubular body, the block having a plurality of ball receptacles; and a retainer plate coupled to the block, the retainer plate having a plurality of ball receptacles, each ball receptacle aligned with one of the plurality of ball receptacles of the block; wherein one of the plurality of balls is rotatably disposed in each pair of aligned ball receptacles.
 20. The centralizer of claim 19, wherein each block, the retainer plate coupled thereto, and the balls disposed therebetween forms a roller ball assembly, the roller ball assembly having an inner surface and an outer surface, and wherein each ball disposed in the roller ball assembly extends through an opening in the outer surface to rotatably engage the outer tubular, whereby the centralizer moves relative to the outer tubular.
 21. The centralizer of claim 20, wherein each ball disposed in the roller ball assembly extends through an opening in the inner surface to rotatably engage the inner tubular, whereby the centralizer moves relative to the inner tubular. 